1. Field of the Invention
The present invention relates to a method of controlling the performance of a thermionic tube having a cathode, a plate and a grid. The present invention also relates to an apparatus for controlling the performance of a thermionic tube when amplifying an audio signal and to an audio signal amplifier.
2. Description of the Related Art
Despite advances in solid state technology for the amplification of audio signals, specialist markets continue to exist for the deployment of thermionic devices, in which amplification is achieved by controlling the flow of electrons in an evacuated tube. Such devices are usually referred to as thermionic tubes or thermionic valves which will be referred to herein as thermionic tubes.
Thermionic tubes continue to be used in high quality audio amplifiers in which to obtain optimum performance, a triode thermionic tube may be arranged to operate in a class A configuration, in which a bias signal is applied such that a single valve may be responsive to both the positive and negative half cycles of an incoming audio signal. An optimum level of bias may be selected during the manufacture of the amplifier and again this bias signal may be adjusted periodically. However, it is known that thermionic tubes degrade through operation therefore after a period of use, although being perfectly functional, degradation may have occurred such that a previous optimum bias level may have become less than optimum for the current operational characteristics of the tube. Thus, in order to maintain optimum performance, it would be preferable for the bias level to be adjusted on a regular basis throughout the lifetime of the tube. However, currently, such an approach would be unrealistic except for very high quality professional applications.
It is also known for thermionic tubes to be used in amplification systems for musical instruments and in particular for electric guitars, including electric base guitars. Some amplifiers of this type operate in class A mode but the majority operate in class B, in which one tube handles the positive half cycle and a co-operating tube deals with the negative half cycle of the input audio signal. To improve linearity it is also known to operate in class A/B mode, thereby obtaining a compromise between the linearity of class A and the power saving characteristics of class B.
It has become standard practice in guitar amplifiers for the tubes to be overdriven well beyond there recommended operating conditions, in which the resulting distortion is embraced as enhancing the overall musical effect; the amplification system effectively becoming part of the instrument. A consequence of driving thermionic tubes to their limits in guitar amplifiers is that the tubes themselves rapidly become degraded and when not actually being played it would be desirable for measures to be taken to ensure that the tubes are not unnecessary forced to work when an output signal is not required. However, presently, except for placing a guitar amplifier in a standby condition, which usually removes the high tension (HT) supply to the tubes, no systems exist for monitoring the performance of the tubes and adapting a working environment, so as to enhance their performance characteristics while at the same time limiting unnecessary damage.